This application claims right of benefit of prior filing date of Japanese Patent Application No. H13-305521 (2001), filed Oct. 1, 2001, entitled xe2x80x9cImaging Optical System and Exposure Apparatus,xe2x80x9d the content of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention pertains to an imaging optical system and exposure apparatus which may be favorably used during manufacture for example of semiconductor elements, liquid crystal display elements, image pickup elements, CCD elements, thin-film magnetic heads, and/or other such microdevices through use of photolithographic techniques.
2. Background of the Invention
Minimum linewidths in circuit patterns grow smaller with each successive generation of technology in the semiconductor and like fields of art. With each such generation of technology, greater resolving power has therefore been demanded of the exposure apparatuses used in such fields of art, and the light used for exposure (hereinafter xe2x80x9cactinic lightxe2x80x9d) has moved to shorter and shorter wavelengths. Note that except where otherwise specified, xe2x80x9clightxe2x80x9d and xe2x80x9cradiationxe2x80x9d are used interchangeably herein and without intention to limit either to wavelengths which are visible or invisible or the like, both terms being used to refer to that portion of the electromagnetic spectrum where wavelength is less than about 1 mm and in particular including a range from roughly the infrared to the x-ray region, inclusive. xe2x80x9cActinicxe2x80x9d light or radiation as used herein refers to light or radiation used for exposure without regard to whether such exposure occurs by a chemical, physical, or other process. xe2x80x9cExposurexe2x80x9d as used herein refers to any change due to receipt of such actinic light or radiation at the wafer or other such substrate or workpiece.
One candidate under consideration as a next-generation exposure apparatus is an EUVL (Extreme Ultra Violet Lithography) apparatus making use of EUVL technology and employing light of wavelengths on the order of 5 nm to 20 nm. Because there are no materials having transmittances that would allow adequate formation of an optical system from refractive elements at this range of wavelengths, such optical systems must be constructed exclusively from reflective surfaces. Many types of optical systems have been proposed as EUVL projection optical systems.
From the laws of physics, the limit of resolution of an exposure apparatus is given by the following formula:
(Linewidth at limit of resolution)=kxc3x97xcex/NA 
. . . where xcex is actinic light wavelength, NA is the numerical aperture of the projection optical system, and k is a constant which depends on the characteristics of the apparatus and photosensitive resin, as well as on various other conditions. The laws of physics establish a minimum value of 0.25 for k, but practical considerations associated with actual apparatuses make 0.4 a more reasonable value for k. At present, the wavelength of light employed by most EUVL apparatuses is 13.4 nm. At the present time, EUVL apparatuses are typically contemplated for use in applications demanding minimum linewidths on the order of 50 nm. Using these values and the above formula, it can be determined that the projection optical system of an EUVL apparatus capable of achieving minimum linewidths of 50 nm should have an NA of
(Required NA) greater than kxc3x97xcex/(minimum linewidth)=0.4xc3x9713.4/50=0.11 
Moreover, to accommodate minimum linewidths of 30 nm, which represents the generation after the 50 nm minimum linewidth generation, an NA of 0.18 or higher would be required.
And yet, of the many EUVL optical systems proposed to date, there is virtually none which achieves an NA on the order of 0.1 to 0.18 over an exposure area of practical size while at the same time permitting adequate correction of aberration. This is because until recently, EUVL was only contemplated for use in applications demanding minimum linewidths of 100 nm. The recent thinking is that the 100 nm minimum linewidth generation and the 70 nm minimum linewidth generation can be accommodated by exposure technologies other than EUVL. This has consequently ratcheted forward the generation contemplated for handling using EUVL technology, but the truth of the matter is that as of the present time there are but few optical systems which have the resolving power capable of accommodating minimum linewidths of 50 nm and smaller. Of the several types of optical system proposed to date, while there are extremely few cases where aberration is adequately corrected in the context of an NA on the order of 0.1 to 0.18 in a system whose design parameters have been disclosed, some examples are disclosed at Japanese Patent Application Publication Kokai No. H10-90602 (1998) and Japanese Patent Application Publication Kokai No. H9-211332 (1997).
Now, with EUVL, because the pattern to be transferred is formed on a mask which is used in reflection, illuminating light must be incident on the mask at an oblique angle with respect thereto. This is because if illuminating light were incident perpendicularly on a reflection mask, the optical path of an illumination system which illuminates the mask and the optical path of an imaging system which is arrived at after reflection from the mask would overlap, and optical elements of the illumination system would occlude the optical path of the imaging system, and/or optical elements of the imaging system would occlude the optical path of the illumination system.
In causing a light beam to be obliquely incident thereon, both of the foregoing two patent publications disclose that the light beam be inclined in such a direction so as to cause it to be constricted as it goes from the mask surface to the projection optical system. Or stating this another way, the entrance pupil of the projection optical system is to the projection optical system side of the mask.
The basis for being able to state this in either of the foregoing two ways will be explained through use of a drawing. In a projection optical system or other such optical system without vignetting, the entrance pupil is defined as the image of the applicable aperture stop as produced by that portion of the optical system between the object surface and that aperture stop. Now, a ray passing through the center of the aperture stop is called a principal ray, and from the foregoing definition it is clear that the entrance pupil will be located at the intersection of the optical axis and an extension of a principal ray passing through the image plane. This is shown diagrammatically in FIG. 6.
FIG. 6(a) shows an example of a design solution in which a light beam is inclined in such direction as to cause it to be constricted with respect to the optical axis as it goes from the mask surface (object surface) to a projection optical system. In such a case, the intersection of the principal ray and the optical axis will be to the right, or to the projection lens side, of the object plane, and the entrance pupil will be to the projection optical system side of the mask plane (object plane). Conversely, in FIG. 6(b), a light beam is inclined in such direction as to cause it to diverge with respect to the optical axis as it goes from the mask surface (object surface) to the projection optical system, in which case the intersection of the principal ray and the optical axis will be to the left, or on the side opposite the projection lens side, of the object plane, and the entrance pupil will be on the side opposite the projection optical system side of the mask plane (object plane).
The former situation (i.e., a situation such as that shown in FIG. 6(a)) creates the following difficulties from the standpoint of illumination optical system design. For efficient, distortion-free transfer of the mask pattern to the surface being exposed it is, in general, required that the exit pupil of the illumination optical system be conjugate to the entrance pupil of the projection optical system. The location of the entrance pupil of the projection optical system must therefore be taken into consideration during illumination optical system design. FIG. 5(a) and (b) show schematic diagrams of illumination systems designed, respectively, for the situation where the entrance pupil of the projection optical system is located to the projection optical system side of the mask plane and for the situation where the entrance pupil of the projection optical system is located to the illumination system side of the mask plane. FIG. 5(a) and (b) both show the optical path of a light beam which exits a light source 1 and is reflected from a reflective optical system 2 to illuminate a mask 3. Here, AX indicates the optical axis.
As is clear from FIG. 5(a), when the entrance pupil of the projection optical system is located to the projection optical system side of the mask plane, a light beam directed toward reflective optical system 2 from light source 1 will approach a light beam reflected from reflective optical system 2 and directed toward mask 3. Such beams may even overlap in some cases, and isolation of the beams is difficult. On the other hand, as is clear from FIG. 5(b), when the entrance pupil of the projection optical system is located to the illumination system side of the mask plane, there is less opportunity for such overlapping of light beams, facilitating isolation of light beams. Based on the foregoing, it is clear that a design locating the entrance pupil of the projection optical system to the projection optical system side of the mask plane will have the disadvantage that illumination system design will be difficult.
Problems arising when known illumination systems are combined with projection optical systems described in the foregoing two patent publications will now be explained. Among the various types of EUVL illumination systems proposed to date, a system described in a paper entitled xe2x80x9cHigh-Efficiency Condenser Design for Illuminating a Ring Fieldxe2x80x9d by William C. Sweatt (OSA Proceedings on Soft X-Ray Projection Lithography Vol. 18, 1993) is typical of those under consideration in the U.S. Furthermore, a system which has recently received considerable attention was introduced in a paper entitled xe2x80x9cA Novel Condenser for EUV Lithography Ring-Field Projection Opticsxe2x80x9d by Henry N. Chapman and Keith A. Nugent (part of the SPIE Conference on EUV, X-Ray, and Neutron Optics and Sources, Proceedings of SPIE Vol. 3767, July 1999).
In these illumination systems, the exit pupil of the illumination system is located to the illumination system side of the mask plane. In the projection optical systems described in the foregoing two patent publications, the entrance pupil of the projection optical system is to the projection optical system side of the mask. Some optical means is therefore required to make the exit pupil of the illumination system conjugate to the entrance pupil of the projection optical system. In the illumination systems of the foregoing papers, a spheroidal mirror arranged immediately in front of the mask serves as such optical means.
However, a spheroidal mirror is an optical element that, while completely aberration-free with respect to imaging between the two foci thereof, nonetheless produces an extremely large amount of aberration for imaging between a point even slightly removed from a focus thereof and a point conjugate thereto. While this would present no problem for a theoretical point light source, real light sources will, in general, have some extension in space. Such method therefore causes introduction of distortion in the illuminating light, distorting the image of the light source formed at the pupil of the projection optical system and making distortion-free transfer of the mask pattern to the surface being exposed difficult.
The present invention was conceived in light of such problems and has as its object the provision of an imaging optical system and exposure apparatus which are not difficult to design, which permit sufficiently large NA as well as satisfactory correction of aberration, and which, when combined with an illumination system, permit establishment of a conjugate relationship between the exit pupil of the illumination system and the entrance pupil of the projection optical system.
As described in further detail below, the present invention permits an imaging optical system to be provided which has sufficiently high numerical aperture and which permits satisfactory correction of aberration. Furthermore, when used in combination with illumination systems, the present invention permits an imaging optical system to be provided which facilitates illumination system design, making it possible to ensure that exit pupils of illumination systems are made conjugate to entrance pupils of projection optical systems without the need to employ special optical means. Moreover, when such imaging optical systems are used to carry out exposure, the present invention permits an exposure apparatus to be provided which is capable of projecting images of extremely detailed patterns onto substrates at high resolution. It should be noted that while the plural form of certain elements is used in this disclosure, the present invention may also use the singular form of an element. For example, there may be only one exit pupil or one entrance pupil.
In order to solve one or more of the foregoing problems and/or achieve one or more of the foregoing objects, an imaging optical system in accordance with a first aspect of the present invention, in the context of a catoptric imaging optical system capable of forming substantially in one or more second planes one or more complete and/or partial images of one or more objects substantially in one or more first planes, is characterized in that the system has at least one species selected from among the group consisting of one or more entrance pupils, at least one of which is located substantially on an opposite side of at least one of the first plane or planes from the imaging optical system exclusive of the at least one entrance pupil so located. The optical system further has one or more exit pupils, at least one of which is located substantially on an opposite side of at least one of the second plane or planes from the imaging optical system exclusive of the at least one exit pupil so located.
Here, because the first and second planes are conjugate with respect to each other, it is possible to exchange object planes and image planes, in which case the foregoing is equivalent to saying that complete and/or partial images of objects substantially in second planes may be formed substantially in first planes. In the event that second planes are thus taken to be object planes, the foregoing xe2x80x9cone or more exit pupils, at least one of which is located substantially on an opposite side of at least one of the second plane or planes from the imaging optical system exclusive of the at least one exit pupil so locatedxe2x80x9d is equivalent to one or more entrance pupils, at least one of which is located substantially on an opposite side of at least one of the second plane or planes from the imaging optical system exclusive of the at least one entrance pupil so located.
If such an imaging optical system is used as a projection optical system in combination with at least one illumination system having at least one exit pupil to the illumination system side of at least one plane being illuminated, in order to make the exit pupil of the at least one illumination system conjugate to the entrance pupil of the projection optical system, it is sufficient to simply locate the entrance pupil of the projection optical system, such that it is coincident with the exit pupil of the illumination system. One or more embodiments of the present invention may thus make it possible to dispense with special optical means such as a spheroidal mirror, and may moreover permit avoidance of problems attendant upon use of such optical means.
An imaging optical system in accordance with a second aspect of the present invention, in the context of a catoptric imaging optical system capable of forming substantially in one or more second planes one or more complete and/or partial images of one or more objects substantially in one or more first planes, is characterized in that the system has one or more entrance pupils. At least one of the entrance pupils is located substantially on an opposite side of at least one of the first plane or planes from the imaging optical system exclusive of the at least one entrance pupil so located. The imaging optical system is substantially telecentric on at least a same side thereof as at least one of the second plane or planes.
If such an imaging optical system is used as projection optical system in an exposure apparatus in combination with at least one illumination system having at least one exit pupil to the illumination system side of at least one plane being illuminated, in order to make the exit pupil of the at least one illumination system conjugate to the entrance pupil of the projection optical system, it is sufficient to simply locate the entrance pupil of the projection optical system such that it is coincident with the exit pupil of the illumination system. One or more embodiments of the present invention may thus make it possible to dispense with special optical means such as a spheroidal mirror, and may moreover permit avoidance of problems attendant upon use of such optical means. In addition, by making the system telecentric on the second plane side, because the size of the circuit pattern or the like being projected will remain unchanged despite change in focus positioning of planes of exposure and/or projection optical systems, this may be favorably employed in an exposure apparatus.
An imaging optical system in accordance with a third aspect of the present invention, in the context of a catoptric imaging optical system capable of forming substantially in one or more second planes one or more complete and/or partial images of one or more objects substantially in one or more first planes, is characterized in that the system includes one or more exit pupils. At least one of the exit pupils is located substantially on an opposite side of at least one of the second plane or planes from the imaging optical system exclusive of the at least one exit pupil so located. The imaging optical system being substantially telecentric on at least a same side thereof as at least one of the first plane or planes. In such constitution, exchanging object side and image side of the imaging optical system permits attainment of action and benefit similar to those attained in the imaging optical system in accordance with the second aspect of the present invention.
An imaging optical system in accordance with a fourth aspect of the present invention, in the context of an imaging optical system according to any one of the first through third aspects of the present invention, is characterized in that it further includes one or more locations disposed in one or more optical paths from at least one of the first plane or planes to at least one of the second plane or planes, inclusive, and substantially optically conjugate to at least one of the first plane or planes. Employment of an optical system of this type facilitates attainment of optical system designs having image-side NAs of 0.11 or higher.
An imaging optical system in accordance with a fifth aspect of the present invention, in the context of an imaging optical system according to any one of the first through fourth aspects of the present invention, is characterized in that all of the reflective surface or surfaces of the imaging optical system are located in one or more spaces substantially between at least one of the first plane or planes and at least one of the second plane or planes, inclusive. Such constitution permits prevention of complicated optical system designs and of increases in size of the apparatus, and permits facilitation of optical system adjustment and of manufacture of an apparatus employing such optical system.
An exposure apparatus in accordance with a sixth aspect of the present invention is characterized in that it includes one or more imaging optical systems according to any one of the first through fifth aspects of the present invention arranged substantially in one or more optical paths substantially between at least one of the first plane or planes and at least one of the second plane or planes, inclusive. The exposure apparatus is capable of transferring to one or more photosensitive substrates, arranged substantially in at least one of the second plane or planes, one or more complete and/or partial images of one or more prescribed patterns formed on one or more masks arranged substantially in at least one of the first plane or planes. The apparatus further has one or more radiation sources, and one or more illumination optical systems arranged substantially in one or more optical paths between at least one of the radiation sources and at least one of the first plane or planes, inclusive, and capable of illuminating at least one of the mask or masks.
Furthermore, the present invention, in the context of an imaging optical system capable of forming substantially in one or more second planes one or more complete and/or partial images of one or more objects arranged substantially in one or more first planes from at least a portion of light reflected from at least one of the first plane or planes, is characterized in that the system comprises one or more entrance pupils, at least one of which is located substantially on an opposite side of at least one of the first plane or planes from the imaging optical system exclusive of the at least one entrance pupil so located. In such a constitution it is preferred that the imaging optical system be constructed so as to be substantially telecentric on at least a same side thereof as at least one of the second plane or planes. In addition, it is preferred that the imaging optical system have a reducing magnification (i.e., have magnification less than unity).
Furthermore, the present invention, in the context of an exposure method for using one or more imaging optical systems according to the first aspect of the present invention to transfer to one or more photosensitive substrates arranged substantially in at least one of the second plane or planes one or more complete and/or partial images of one or more prescribed patterns formed on one or more masks arranged substantially in at least one of the first plane or planes, is a method including an operation wherein radiation is supplied. The method further includes an operation wherein at least one of the mask or masks is illuminated with at least a portion of the supplied radiation, and an operation wherein at least one of the imaging optical system or systems is used to form on at least one of the photosensitive substrate or substrates at least one of the complete and/or partial image or images.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.